The present invention relates to the making of articles from high alloy powders, and in particular tool steel powders and cobalt based hard metal powders. By "powder" in this context is meant alloys in particulate, granular, or powder form. The powder may optionally be blended with minor additions of metal oxide powders or non-metallic powders.
The tool steel powder is first compacted under a high pressure to form a compact in which the powder particles are locked together mechanically. The resulting compact, which has a reasonably high strength, approximates in shape to the final product but may be machined closer to the desired shape subsequently. The compact however does have significant porosity and requires further treatment to bring it to a fully dense form. That further treatment usually consists of sintering the compact, with or without the application of pressure, i.e., the raising of the temperature sufficiently high to bring some at least of the components of the powder into liquid phase to fill such pores as exist in the compact. Provided the sintered compact has a uniform and or high relative density, the final product retains high dimensional accuracy and has mechanical properties indistinguishable from a conventionally made product.
Particularly when the powder has been made by water atomization of a melt, the powder has an undesirably high oxygen content, even when measures are taken to minimize surface oxidation during and subsequent to atomization. The presence of oxygen in combined form in the compact or in the final product has a deleterious effect, resulting for example in inferior strength.
In the past, attempts have been made to reduce the oxygen content by subjecting the powder, prior to compacting, to heat treatment aimed at deoxidizing the powder. Thus Matt et al. (U.S. Pat. No. 3,744,993) suggest deoxidizing tool steel powder at about 1750.degree. - 1875.degree. F in hydrogen to bring the oxygen content of the powder to 0.30 to 0.25%; no further reduction in oxygen content is proposed as the presence of the stated proportion of oxygen was thought to be beneficial. Matt et al also propose that the compact is sintered in a hydrogen or carbonaceous atmosphere; a vacuum atmosphere is deprecated because it was thought that in vacuum significant vaporization of contained chromium occurred, to the detriment of the final product.
Contrary to Matt el al. we have found that
a. for proper mechanical properties in the finished product, the oxygen content should be reduced well below the figure of 0.25% and the required degree of deoxidation cannot be achieved by treatment of the uncompacted powder in a hydrogen atmosphere;
b. the compact can be safely heat treated at elevated temperature in relatively high vacuum, without significant vaporization of the contained chromium occurring to any damaging extent.
One object of the present invention is to achieve more efficient deoxidation by deoxidizing at higher temperatures. This is achieved by at least completing deoxidizing the powder in its compact form.
Another object is to achieve substantially fully dense articles made from tool steel, alloy steel, stainless steel or cobalt based hard metal powders by substantially densifying the compact during sintering.